Device and method for sensing a position of a probe

ABSTRACT

A device for sensing a position of a probe relative to a reference medium, the probe comprising a heater element with a temperature dependent electrical resistance and being adapted to determine probe position by measuring a parameter associated to a thermal relaxation time of the heater element.

TECHNICAL FIELD

The present invention relates to a method and a device for sensing aposition of a probe relative to a reference medium.

BACKGROUND OF THE INVENTION

In the field of the present invention, micro electromechanical systems(MEMs) are included being the technology of mechanical components on themicrometer size, which includes three dimensional lithographic featuresof various geometries. They are typically manufactured using planarprocessing similar to semiconductor processes such as surfacemicromachining and/or bulk micromachining. MEMS are often fabricatedusing modified silicon fabrication technology, molding and plating,electro-discharge machining and other technologies capable ofmanufacturing very small devices. The field of the present inventionalso embraces techniques that use nanometer-sized tips for imaging andinvestigating the structure of materials down to the atomic scale. Suchtechniques include scanning tunneling microscopy (STM) and atomic forcemicroscopy (AFM), as disclosed in U.S. Pat. No. 4,343,993 and EP 0 223918 B1.

Based on the developments in scanning tunneling microscopy and atomicforce microscopy, new storage concepts have been introduced over thepast few years that profit from these technologies. Probes having ananoscale tip have been introduced for modifying the topography and forscanning an appropriate storage medium. Data are written as sequences ofbits represented by topographical marks, such as indentation marks andnon-indentation marks. The tips comprise apexes with a nanometer-sizeddiameter and the indentation marks have a comparable diameter, forexample, a diameter in the range of 30 to 40 nm. Hence, these datastorage concepts promise ultra-high storage area density.

In STM, a nanometer-sized tip is scanned in close proximity to asurface. The voltage applied therebetween gives rise to a tunnel currentthat depends on the tip-surface separation. From a data-storage point ofview, such a technique may be used to image or sense topographic changeson a flat medium that represent a stored information in logical “0”s and“1”s. In order to achieve a reasonably stable current, the tip-sampleseparation must be maintained extremely small and fairly constant. InSTM, the surface to be scanned needs to be a conductive material.

In AFM, the tip rests on one end of a soft spring cantilever. When thetip is brought in close proximity to a surface, resultant forcestherebetween cause bending of the spring cantilever and so may besensed.

A storage device for storing data based on the AFM principle isdisclosed in “The millipede—more than 1,000 tips for future AFM datastorage” by P. Vettiger et al., IBM Journal Research Development, Vol.44, No. 3, March 2000. The storage device has a read and write functionbased on a mechanical x-, y-scanning of a storage medium with an arrayof probes each having a tip. During operation, the probes scan anassigned field of the storage medium in parallel. In this way, high datarates may be achieved. The storage medium comprises apolymethylmethacrylate (PMMA) layer. The nanometer-sized tips are movedacross the surface of the polymer layer in a contact mode. The contactmode is achieved by applying small forces to the probes so that the tipsof the probes can touch the surface of the storage medium. For thispurpose, the probes comprise cantilevers which carry the tips on theirend sections. Bits are represented by indentation marks ornon-indentation marks in the polymer layer. The cantilevers respond tothese topographic changes in the surface while they are moved across it.

Indentation marks are formed on the polymer surface by thermomechanicalrecording. This is achieved by heating a respective probe with a currentor voltage pulse during the contact mode in a way that the polymer layeris softened locally where the tip touches the polymer layer. The resultis an indentation, for example, having a nanoscale diameter, beingformed in the layer.

Reading is also accomplished by a thermomechanical concept. The heatercantilever is supplied with an amount of electrical energy, which causesthe probe to heat up to a temperature that is not high enough to softenthe polymer layer as is necessary for writing. The thermal sensing isbased on the fact that the thermal conductance between the probe and thestorage medium, especially a substrate on the storage medium, changeswhen the probe is moving in an indentation as the heat transport is inthis case more efficient. As a consequence of this, the temperature ofthe cantilever decreases and hence, its resistance changes. This changeof resistance is then measured and serves as the measuring signal.Reading and/or writing the marks is accomplished by moving the probesrelative to the storage medium in lines within a track and moving to thenext track when the end of the respective line has been reached.

It is a challenge to provide a device and method for sensing a positionof a probe.

SUMMARY OF THE INVENTION

According to an embodiment of a first aspect of the present invention,there is provided a device for sensing a position of a probe relative toa reference medium, the probe comprising a heater element with atemperature dependent electrical resistance, the device being operableto determine the position by measuring a parameter associated with athermal relaxation time of the heater element.

An embodiment of the present invention has the advantage of improvedresponse properties and may be distinguished by superior 1/f noise andsuperior drift properties when compared to a previously-proposed methodentailing direct measurement of a heater element's temperature via aresistance measurement of the heater. The present invention is furtherdistinguished in that a translation of an analog position signal into adigital time domain is enabled. This may render it unnecessary tomeasure small analog signals on an absolute scale. Additionally, thismay further substantially reduce manufacturing tolerance-relatedsensitivity and accuracy problems.

An embodiment of the first aspect may comprise a relaxation oscillatorcircuit, the relaxation oscillator circuit being operable to apply abias voltage potential to the heater element.

The relaxation oscillator circuit is distinguished by a simple circuitdesign. In contrast, previously-proposed techniques entailing directelectrical resistance measurements require complex circuitry, forexample, linear amplifiers.

The relaxation oscillator circuit may further comprise a currentthreshold switching unit that is operable to perform a first switchingact, the first switching act comprising switching the bias voltagepotential from a first bias voltage potential to a second bias voltagepotential when a first current threshold is reached and to perform asecond switching act, the second switching act comprising switching thesecond bias potential to the first bias voltage potential when a secondcurrent threshold is reached.

By way of the current threshold switching unit, the probe position withrespect to a reference medium can be determined with simple circuitry.

The current threshold switching unit may be configured to performmultiple of the first and second switching acts consecutively.

This has the advantage that, when the electrical current of the heaterelement attains one of the current threshold values, this automaticallytriggers the current threshold switching unit to perform a switching actso that the other of the current threshold values is reached. This cycleis continuously repeated until terminated by external user intervention.

In the above case, the parameter is preferably a switching frequency ofthe switching unit.

Use of the switching frequency to determine the probe position rendersbasically a digital signal and results in minimal effort to derive theposition of the probe. Depending on the operating point, this alsoensures a reduced noise level, especially with respect to 1/f noise.

Alternatively, the current threshold unit may be configured to performone of the first and second switching acts. In this case, the parameteris preferably a duration of time from applying one of the first andsecond bias voltage potentials to reaching the corresponding first orsecond electrical current thresholds.

In this case, the current threshold switching unit is configured toterminate operation once an electrical threshold is reached. Forexample, if the switching unit is configured to perform the firstswitching act, then once the first electrical current threshold isreached, operation of the relaxation oscillator is terminated. Only ifan external trigger is provided, operation is resumed and the next stageof operation is conducted, that is, the second switching act isperformed. In this case, the probe position above the storage medium canbe obtained by measuring the duration of time taken to attain thecurrent threshold value corresponding to the switching act that isperformed. This mode of operation—the so-called single shot mode isespecially advantageous if the determination of the probe position issynchronized with non-repetitive trigger events.

In the present embodiment, the first current threshold may be greaterthan a steady state current corresponding to the first bias voltagepotential and the second current threshold is less than a steady statecurrent corresponding to the second bias voltage potential and the firstbias voltage potential is greater than the second voltage potential.

The steady state electrical current is the electrical current, which isfinally reached after changing a bias voltage potential if therespective position of the probe was not changed. Values for the firstand second current thresholds and first and second bias voltagepotentials are chosen to ensure efficient and accurate operation of thefirst embodiment.

According to another embodiment of the first aspect of the presentinvention, the device may comprise an LC-resonating circuit, thecapacitive element thereof being the heater element, and wherein theparameter is a resonance frequency of the LC-resonating circuit.

Such a device is in particular practical for low frequency applications.

According to yet another embodiment of the first aspect of the presentinvention, the device may comprise an RC delay line oscillator circuitand wherein the parameter is an oscillation frequency of the RC delayline oscillator.

The advantage of the RC delay line oscillator is that no inductor isneeded.

According to yet another embodiment of the first aspect of the presentinvention, the device may comprise a bridge type oscillator circuit andwherein the parameter is an oscillation frequency of the bridge typeoscillator.

The bridge type oscillator provides superior stability and phase noiseperformance.

According to yet another embodiment of the first aspect of theinvention, wherein each one of the above-described relaxation oscillatorcircuit, LC-resonating circuit, RC delay line oscillator and bridge-typeoscillator further comprises a phase-locking unit for synchronising thecircuit to an external clock. In this way, position informationcontained in a control signal may be used for adjusting the frequency ofthe circuit in order to reduce a phase difference to the referencefrequency. In this case, it is preferable that the parameter is a phasecontrast signal output from the phase-locking unit. This is inparticular advantageous if a mark representing logical information ispresent in the reference (storage) medium and the probe is scanned overthe reference medium. In this case, the external clock signal and theexact mark position need not perfectly match, which makes the sensingmore robust against fluctuations in the clocking signal.

Advantageously, the external clock may be operable to create a clockingsignal corresponding to consecutive mark positions on the referencemedium when the surface thereof is scanned by the probe.

In this way, a simple synchronization with mark positions is possible.There is only a need for a low sampling frequency and one may obtain atthe same time high detection reliability.

Furthermore, a device embodying the first aspect of the presentinvention may be operable to control an overall loop gain of each one ofthe relaxation oscillator circuit, LC-resonating circuit, RC delay lineoscillator circuit and bridge-type oscillator circuit so that the timeit takes to establish a phase match is sufficiently long for a balancestring of marks to be probed.

Marks are in this case preferably represented by indentations and nonindentations. Whenever an indentation is probed during the time intervalbetween two clocking pulses, the phase of the circuit decreases by aconstant amount since the thermal relaxation time is slightly smallerthan average and thus the electrical relaxation time is slightly slowerthan average as well. Conversely, the phase of the circuit increases bya constant amount if no indentation is probed between two clockingpulses. Hence, the digital information probed by the heater element isencoded in an integrated fashion in the phase signal, which then may beanalyzed respectively.

In an embodiment of the first aspect of the present invention, theheater element is operated in a plateau region of a current/voltageinterrelationship curve of the heater element.

In this way, 1/f noise may be reduced as an electrical relaxation time,also referred to as electrical time constant, of the heater element isthen substantially independent from the operating point and onlyreflects the actual thermal relaxation time of the heater element.

According to an embodiment of a second aspect of the present invention,there is provided a method for sensing a position of a probe relative toa reference medium, the probe comprising a heater element with atemperature dependent electrical resistance comprising the step ofmeasuring a parameter associated to a thermal relaxation time of theheater element.

The method aspect corresponds to the device aspect and embodimentsthereof of the present invention.

Any of the device features may be applied to the method aspect of theinvention and vice versa. Features of one aspect may be applied to anyother aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the followingdrawings, in which:

FIG. 1 is a perspective view of a storage device;

FIG. 2 is a cross-sectional view of part of the storage device shown inFIG. 1;

FIG. 3 shows a probe arranged in the storage device of FIG. 1, the probebeing positioned above an indentation-free area of the storage medium;

FIG. 4 shows the probe lowered in an indentation on the storage mediumof device of FIG. 1;

FIG. 5 is a plot of the electrical resistance of the heater element as afunction of its temperature;

FIG. 6 illustrates a current/voltage interrelationship of the resistanceof the heater element;

FIG. 7 is an electrical equivalent circuit of the heater element;

FIG. 8 is a circuit diagram of a relaxation oscillator embodying thepresent invention;

FIG. 9 is a current/voltage diagram corresponding to the relaxationoscillator shown in FIG. 8;

FIG. 10 is a circuit diagram of an LC-resonating circuit embodying thepresent invention;

FIG. 11 is a circuit diagram of an RC delay line oscillator embodyingthe present invention;

FIG. 12 is a circuit diagram of a bridge type oscillator embodying thepresent invention;

FIG. 13 is a circuit diagram of an oscillator with a phase locked loopunit;

FIG. 14 shows a plot of the heater element, I_HE, and heater elementpotential difference, U_HE, respectively plotted as a function of time,t, during operation of the relaxation oscillator shown in FIG. 8; and

FIG. 15 respectively illustrates the power spectral densities of anembodiment of the present invention and a previously-proposed technique.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a storage device, in particular forstoring data. The storage device comprises a storage medium 2 that ispreferably formed by a polymer layer. The storage medium 2 in this caseembodies a reference medium. The polymer layer is preferably formed of apolymethylmethacryllate (PMMA) layer. The storage medium 2 may, however,also consist of different materials like an electrically conductivematerial. The storage medium 2 is supported by a substrate 4. Thesubstrate 4 is preferably formed of silicon.

An array of probes 6 is mounted on a common frame 8. Only a few probesare shown in FIG. 1. The storage device may, however, comprise moreprobes 6, such as 1024 or an even significantly larger number. It may,however, also just comprise one or a few probes 6.

Each probe 6 comprises terminals, each of which are electricallyconnected to a control and information processing unit 12 viaelectrically conductive lines. The terminals may all be individuallyconnected to the control and information processing unit 12. However, inorder to reduce the complexity of the wiring, the terminals of theindividual probes 6 may also be connected via rowlines and column-linesand possibly via multiplexers to the control and information processingunit 12.

The control and information processing unit 12 is designed for creatingcontrol parameters applied to the probes 6 via the terminals or sensingparameters present on the terminals, such as a current or a voltagepotential for a write or read mode. The nature of the control parametersand the sensing parameters depends on the nature of the storage medium2. The control and information processing unit 12 is also a device forsensing a position of the probe 6 relative to the storage medium 2. Thecontrol and information processing unit 12 is further designed forcontrolling a movement of the storage medium 2 and the substrate 4relative to the frame 8 in an x-,y- and z-direction. The actuation forthis is accomplished by a scanner 18.

Hence, the storage medium 2 and the frame 8 containing the probes 6 aremovable relative to each other. In an alternative embodiment, thescanner 18 may act on the frame 8 in order to move the frame 8 in thex-, y- and z-direction relative to the storage medium 2 and thesubstrate 4. A relative movement in a z-direction may also beaccomplished by applying respective electrostatic forces on the probes 6possibly containing a capacitive platform forming a first electrode of acapacitor and further comprising a second electrode being arranged in afixed position relative to the storage medium 2.

In the storage medium 2, reference numerals 20, 22 and 24 denote marksrepresenting logical information. Preferably, they form topographicmarks and may represent logical “1”s as indentations, whereas theabsence of an indentation may represent logical “0”s. The marks 20, 22and 24 are preferably of a nanoscale diameter. In FIG. 1, only a fewmarks 20, 22 and 24 are shown, which also do not represent their realphysical properties. However, it can be appreciated that the marks 20,22 and 24 may be greater in number and may represent more logic valuesthan two.

In FIG. 2, a cross-sectional view of the data storage device of FIG. 1is shown. Part of a probe 6 is shown. The probe 6 comprises a springcantilever 26 on which a tip 28 is mounted, the tip 28 having an apex30, which preferably has a nanometer-sized radius. The marks 20, 22 and24, shown as indentation marks, are formed by pushing the apex 30 of thetip 28 into the storage medium 2.

The probe 6 comprises the spring cantilever 26 and a heater element HEfor performing write/read operations. The heater element HE is formed asa temperature dependent resistor, which is thermally coupled to the tip28. However, it does not need to be thermally coupled to the tip 28,especially if only read operations are performed. It can be appreciatedthat the write/read operations may also be performed by separate heaterelements HE.

The spring cantilever 26 and the heater element HE preferably arefabricated from silicon. Electrically conductive connections to theterminals of the probe consist preferably of highly doped areas of thespring cantilever 26, whereas the heater element HE, in the case ofbeing formed as a temperature dependent resistor, may be formed by lessdoped silicon, which yields a high electrical resistance. The dopingconcentration of the heater element HE may, for example, be in the rangefrom 10¹⁶-10¹⁸ cm⁻³.

In the case of the AFM based storage device, the tip 28 does not need tobe electrically conductive. The tip may also be formed from a differentmaterial, such as a magnetic material.

As can be seen most clearly from FIG. 3, the heater element HE is spacedapart from the storage medium 2 at a distance h, if the apex is in anarea of the storage medium 2 without an indentation, and at a distanceh—delta h if the apex is moved into an indentation (see FIG. 4).

During operation of the storage device, each probe 6 is moved across itsrespectively assigned field. It is moved along lines, each line forminga track. The data, which are represented by marks, are written andrespectively read consecutively along the respective tracks and, at theend of each track, the respective probe 6 is moved to the next track.

A scanning direction is the direction of on track relative movementbetween each probe 6 and the storage medium 2. A cross track directionis the direction perpendicular to the scanning direction.

During the write mode, the probe 6 is scanned in the scanning directionin the line of the respective track. Marks 20, 22 and 24 are created byapplying a heating pulse to the heater element HE via a respectivecurrent or voltage pulse. The heat dissipated from the heater element HEsoftens the storage medium 2 and the tip 28 forms a respectiveindentation at the mark 20, 22 and 24, if the spring cantilever 26 isimposed with a respective force. This force may be inherent to themechanical properties of the spring cantilever 26. It may, however, alsobe created in another way, such as, for example, by an electrostaticforce. In this case, a respective force pulse is imposed to therespective capacitor by charging the capacitor respectively.Alternatively, only a suitably chosen force pulse may be provided ifcold writing should be achieved. The heating pulse and also the forcepulse are time-synchronized in order to achieve a desired on-trackdistance between adjacent marks 20, 22 and 24. For that purpose, aclocking signal is used, which is created by a clocking signal unit ofthe control and information processing unit 12.

During a read mode, the probe 6 is scanned in the scanning directionalong each line of the respective track. When the tip 28 reaches anindentation representing a mark 20, 22 and 24, the tip 28 moves into therespective indentation as the tip 28 is pressed onto the storage medium2 with a given spring force. This movement into the respectiveindentation may then be sensed and in that way a respective indentationmay be identified.

The principal of thermo-mechanical sensing, in the context of thepresent invention, is now described in further detail below.

The heater element HE is brought into proximity to the reference medium(typically less than one micrometer relative to the reference medium),which reference medium may be the storage medium 2 and which serves as aheat sinking surface. By electrically energizing the heater element HE,it attains a temperature T_H. The heater element's temperature, T_H, maytherefore be determined by a balance between an electrical power P_ELapplied to the heater element HE and the dissipation of the thermalenergy. One of the dissipation paths is heat flux from the heaterelement HE to the heat sinking surface formed by the storage medium 2and also the substrate 4. The amount of power dissipated through thispath is proportional to a temperature difference of the temperature T_Hof the heater element HE and the storage medium 2. On the other hand,the amount of power is inversely proportional to the distance h betweenthe heater element HE and the heat sink that is the storage medium 2.Hence, with reference to a steady state condition concerning theelectrical power P_EL and the temperature T_H of the heater element HE,the following equation holds:delta T _(—) H=R _(—) th·delta P _(—) EL+eta*T _(—) H·delta h/h   (1)where R_th is the overall thermal resistance of the heater element HEincluding at least the thermally coupled part of the spring cantileverand in general all thermally coupled parts of the probe. Further, italso includes the medium in between the heater element HE and thestorage medium 2. eta is an efficiency parameter, which measures thefraction of heat dissipation to the heat sink and which is typically inthe order of 0.1 to 0.5 depending on the design of the probe 6.

The principle of distance sensing exploits the fact that the electricalresistance R_HE of the heater element HE depends on the heater element'stemperature T_H. FIG. 5 shows a typical plot of the electricalresistance R_HE of the heater element HE as a function of thetemperature T. The electrical resistance R_HE of the heater element HEreaches a maximum value, which is of the order of 2 to 3 times the roomtemperature resistance, at an inversion in temperature T_INV. Theinversion temperature T_INV is on the order of 400 to 650 degreesCelsius depending on the doping concentration. Above the inversiontemperature T_INV, the resistance drops as a result of thermallygenerated charge carriers. As a result of this temperature dependence,non-linear current/voltage interrelationships are obtained as depictedin FIG. 6, with U_HE denoting the heater element potential difference,I_HE denoting the heater element electrical current and I_DC denoting asteady state current obtained in a steady state concerning theelectrical power P_EL provided to the heater element HE and itsposition. A respective curve of the steady state current I_DC isreferred to as current/voltage interrelationship curve. It is clearlyvisible in the current/voltage interrelationship curve that it has aplateau region ranging from around 2 volt to a little below 6 volts anda corresponding current from 0 to about 0.4 milliampere. In FIG. 6, eachpoint of the current/voltage interrelationship curve corresponds to adifferent temperature of the heater element HE under the steady statecondition concerning the distance h and, for the respective point, alsoa corresponding steady state electrical power P_EL, which may howevervary from the respective points in the curve.

It is known to perform position sensing by monitoring the changes of theelectrical operating point delta I_HE of the current change and deltaU_HE of the heater element voltage potential change, which result fromchanges of the heater temperature delta T_H induced by a change of thedistance delta h/h. One of the problems with this direct sensing method,in contrast to a direct translation of electrical properties to thermalproperties, is the fact that current/voltage interrelationships aresubject to erratic variations related to carrier density fluctuationswhich give rise to noise, in particular 1/f type noise and systematicsensing errors associated with aging and temperature changes of thesubstrate, which cause corresponding shifts of the operating point.

The position sensing mechanism according to an embodiment of the presentinvention circumvents these problems by sensing the thermal resistancevia its influence on a thermal relaxation time, tau, of the heaterelement HE. tau is given by the following equation:tau=C _(—) th*R _(—) th   (2)where C_th denotes the heat capacity of the heater element HE. The heatcapacity C_th has the advantage that it remains constant. In analogy toequation 1, one can derive the following equation:delta tau=tau*eta*delta h/h   (3)

Hence, changes of the distance delta h/h translate directly intocorresponding changes of the thermal relaxation time tau independent ofthe electrical operating point of the heater element HE or also theprobe 6 as reflected by the absence of the delta P_EL term in equation3, which is a major source of error in the previously-describeddirect-sensing scheme.

In order to probe the thermal relaxation time tau of the heater elementHE, an electrical measurement is used. For minimal deviations from anoperating point, being defined by minimal deviations from the distance hand the electrical power P_EL supplied, the electrical impedance of theheater element HE may be represented by an equivalent circuit as shownin FIG. 7. R_0 denotes an operating point electrical resistance of theheater element HE and may be derived from FIG. 6. A dimensionlessconstant k is given by:k=(1−R _(—)0·delta I_HE/delta U _(—) HE)/(1+R _(—)0·delta I _(—)HE/delta U_HE)   (4)with a delta I_HE/delta U_HE representing the slope of thecurrent/voltage interrelationship curve at the operating point. In thealready above mentioned horizontal plateau region of the current/voltageinterrelationship curve, the constant k is therefore approximately equalto 1, irrespective of the operating point. Hence, an electrical timeconstant tau_el of the heater element HE is in this region substantiallyindependent from the operating point and only reflects the thermalrelaxation time tau of the heater element HE. The electrical relaxationtime tau_el is given by:tau_el=R _(—)0*C _(—) HE=tau/(2*k)   (5)

The above-stated relationship is exploited in the present invention inorder to determine the probe position above a reference medium.Embodiments according to the present invention will now be described infurther detail below.

FIRST EMBODIMENT

A first embodiment of the present invention comprises a relaxationoscillator circuit, which is shown in FIG. 8. It is, during operation,electrically connected to the heater element HE of the respective probe6. It is arranged in the control and information processing unit 12. Itcomprises voltage sources being designed for generating a first and asecond bias voltage potential V_b1, V_b2, respectively.

The heater element HE may be represented by its electrical equivalentcircuit according to FIG. 7. When operating an electrical heater in thearea of the plateau region of the current/voltage interrelationshipcurve I_DC, the parallel resistance to the electrical capacitance C_HEof the heater element HE may be assumed to be significantly greater thanthe operating point electrical resistance R_0. It is to be noted thatthe operating point may be represented by a current source beingarranged in a parallel fashion to the equivalent circuit according toFIG. 7.

The heater element HE is on one side electrically connected to acurrent/voltage converter 40. The current/voltage converter 40 iselectrically connected to a current threshold detector 42. The currentthreshold detector 42 is preferably designed to compare its inputvoltage, which is representative of the heater element electricalcurrent I_HE, to at least one electrical current threshold, preferablyto two electrical current thresholds, which are referred to as firstelectrical current threshold I_THD1 and second electrical currentthreshold I_THD2. The current threshold detector 42 is part of a currentthreshold switching unit, which further comprises a first switch 44 anda second switch 46. The current threshold detector 42 is furtherdesigned for creating a signal having a switching frequency f_SW.

Operation of the relaxation oscillator circuit shown in FIG. 8 is nowdescribed with the aid of FIG. 9 in the following steps:

-   (i) In FIG. 9, operation starts from a “cold” heater element HE,    i.e. at the origin of the FIG. 9 plot. When the first switch 44 is    closed, the first bias voltage potential V_b1 is applied to the    heater element HE. This causes the electrical current I_HE value to    instantly increase to a value denoted by P0, which value corresponds    to the first bias voltage potential V_b1 divided by the operating    point electrical resistance R_0 of the heating element when the    first switch 44 is closed, i.e. its electrical resistance when it is    cold. As can be seen from FIG. 9, the electrical current I_HE value    at P0 is significantly larger than the steady state current I_DC    corresponding to the first bias voltage potential V_b1;-   (ii) Ramping of the I_HE value in step (i) causes the heater element    HE to gradually heat, this being equivalent to the electrical    capacitance C_HE of the heater element HE being charged. Then, and    in order for a steady state to be attained, the current approaches    the current/voltage interrelationship curve I_DC in an exponential    relaxation mode. At a time corresponding to a point P2, the heater    element electrical current I_HE reaches the first electrical current    threshold I_THD1. This triggers the switching unit to open the first    switch 44 and close the second switch 46. This results in the second    bias voltage potential V_b2 being applied to the heater element HE    and the I_HE value to instantly decrease to a value denoted by P3 in    FIG. 9. The I_HE value at P3 depends on the operating point    electrical resistance R_0 at point P2—it may be obtained as the    current value corresponding to an intersection point, which point    lies on a straight line connecting P2 to the origin of the FIG. 9    plot and corresponds to the second bias voltage potential V_b2. As    can be seen from FIG. 9, at P3, the heater element electrical    current I_HE is significantly smaller than the steady state current    I_DC corresponding to the second bias voltage potential V_b2; and-   (iii) After the electrical current of the heater element drops to    P3, the heater element cools down, which corresponds to the    discharge of the electrical capacitance C_HE. The discharge process    causes a gradual rise in the heater element electrical current I_HE    until, at a point of time corresponding to point P4 on FIG. 9, the    second electrical current threshold I_THD2 is reached. This triggers    the current threshold switching unit to open the second switch 46    and close the first switch 44, which then results in the I_HE being    ramped up to P1. At P1, the heater element electrical current I_HE,    which is given by the first bias voltage potential V_b1 divided by    the operating point electrical resistance R_0 corresponding to point    P4, is significantly larger than the steady state current I_DC    corresponding to the first bias voltage potential V_b1.

The above-described mode of operating the circuit of FIG. 8 hashenceforth been referred to as the steady state relaxation oscillationmode. Furthermore, operation has been shown to start from a cold heaterelement HE in FIG. 9 by way of example, but as can be appreciated,another starting point based on the condition of the bias voltagepotential value being less than V_b1 can be used.

The oscillation frequency of the circuit shown in FIG. 8, which is in afixed relation and proportional to the switching frequency f_SW of theswitching unit, depends on the choice of the first and second biasvoltage potentials V_b1, V_b2 and the first and second electricalcurrent threshold I_THD1, I_THD2. In an embodiment of the presentinvention, values for these 30 variables are chosen in order to enablethe above-explained steady state relaxation oscillation mode. In thepresent embodiment, the first current threshold I_THD1 is chosen to belarger than the steady state current I_DC corresponding to the firstbias voltage potential V_b1 and the second electrical current thresholdI_THD_2 to be less than the steady state current I_DC corresponding tothe second bias voltage potential V_b2. Also, f_SW is adjusted to rangefrom 0.1 times the electrical relaxation time tau_el up to 10 times theelectrical relaxation time tau_el.

The switching frequency f_SW, which is proportional to the electricalrelaxation time tau_el, is also, in the view of equation 5, proportionalto the thermal relaxation time tau. Hence, the switching frequency f_SWcan be used to measure the thermal relaxation time and, in the view ofequation 3, a change in the distance h of the heater element HE withrespect to the storage medium 2. Thus, it can be appreciated that thedistance h from the heater element HE to the storage medium 2 may beobtained in dependence on the switching frequency f_SW. For thispurpose, a characteristic curve or field may be provided, from whichrespective values of the distance h may be derived depending on theswitching frequency f_SW. Alternatively, the distance h may also beobtained dependent on the duration of a time from reaching point P1 toreaching point P2 or from reaching point P3 to reaching point P4 or fromreaching point P0 to reaching point P2.

The above-described embodiment of the present invention may be operatedin two main modes, namely, in continuous mode or single-shot mode.

In the continuous mode, the steady state relaxation mode is continuouslyrepeated, that is, the circuit of FIG. 8 is operated to continuouslyrepeat the cycle in accordance with operating steps (i) to (iii) above.This mode relies on the fact that, when the I_HE value attains one ofthe current threshold values, I_THD1, I_THD2, this automaticallytriggers the switching unit to control the opening/closing of switches44, 46, as appropriate. This mode of operation is continuous untilterminated by external user intervention.

By contrast, in the single shot mode, the circuit of FIG. 8 isconfigured to terminate operation once an electrical threshold isreached. For example, when switch 44 is closed and switch 46 is opened,and once I_THD1 is reached (corresponding to the transition of I_HE fromP0 to P2), operation of the relaxation oscillator is terminated. Only ifan external trigger is provided, operation is resumed and the next stageof operation is conducted, that is, switch 44 is opened and switch 46 isclosed in order to effect transition of I_HE from P3 to P4 so that thevalue I_THD2 is attained. In this mode of operation, the distance h ofthe probe 6 above the storage medium 2 can be obtained by measuring theduration of time taken to attain the current threshold value, forexample, in the above-described case, the distance h can be determinedfrom the duration of time between transition points P0 to P2.

The single shot mode is especially advantageous if the determination ofthe distance h is synchronized with non-repetitive trigger events. Anarbitrary voltage potential may be applied to the heater element HE,such as a reference voltage potential, in this operation mode.

SECOND EMBODIMENT

A second embodiment of an electronic circuit enabling the sensing of theposition of the heater element HE relative to the storage medium 2comprises an LC-resonating circuit and is shown in FIG. 10. Duringoperation of the circuit, the heater element HE is electricallyconnected on one side to the current/voltage converter 40 and on theother hand the heater element is connected to an output of a feedbackamplifier 50. The heater element HE is arranged electrically in parallelto an inductance L.

The output of the feedback amplifier 50 has a signal that isrepresentative of the resonance frequency f_RES_LC of the LC-resonatingcircuit. A Q factor will be of order unity for resonance frequenciescomparable to 1/tau_el, which renders this circuit practical preferablyfor low frequency applications (f_RES_LC<<1/tau_el). The Q factor, orquality factor, is a measure of the “quality” of a resonant system. On agraph of response versus frequency, the bandwidth is defined as the 3 dBchange in voltage level in respect to the resonant frequency. The Qfactor is given by the resonant frequency divided by the bandwidth. Inthe case of the LC-resonating circuit, the Q-factor is given by1/(2Π*f_RES_LC*C_HE*R_0).

In this embodiment, the resonant frequency f_RES_LC is proportional tothe electrical relaxation time tau_el and, hence, the thermal relaxationtime tau. Preferably, the inductance L is chosen to have an inductancevalue of around L=1/((2Π*f_RES_LC)²*C_HE).

THIRD EMBODIMENT

A third embodiment comprises a RC delay line oscillator and is shown inFIG. 11. The electrical resistors R1 and R2 and the capacitors C1 and C2are preferably dimensioned in the order of the operating pointelectrical resistance R_0 and respectively the electrical capacitanceC_HE of the heater element HE. More preferably, they match therespective operating point electrical resistance R_0 and respectivelythe electrical capacitance C_HE of the heater element HE in a range ofat maximum a factor 2 to 3. The advantage is that the resistors R1, R2and the capacitors C1 and C2 do not have to exactly match the operatingpoint electrical resistance R_0 and/or respectively the electricalcapacitance C_HE of the heater element HE in order to obtain a signalthat appropriately measures the thermal relaxation time tau. The RCdelay line oscillator further comprises the current/voltage converter 40and the feedback amplifier 50.

The output of the feedback amplifier 50 is representative of anoscillation frequency f_osc_RC of the RC delay line oscillator. Theoscillation frequency f_osc_RC is representative of the electricalrelaxation time tau_el and, in this way, of the thermal relaxation timetau.

FOURTH EMBODIMENT

A fourth embodiment of the present invention comprises a bridge typeoscillator and is shown in FIG. 12. In this case, the bridge typeoscillator is a type of wien bridge. The resistors R3, R4, R5 and thecapacitor C3 are preferably dimensioned in a similar way as the resistorR_0 and capacitor C_HE. The bridge type oscillator further comprises adifferential amplifier 54 whose inputs are connected electricallybetween the heater element and the resistor R3 and, respectively, theresistor R4 and R5. The output signal of the differential amplifier 54has a frequency that is representative of an oscillation frequencyf_osc_bt of the bridge type oscillator. f_osc_bt is representative ofthe electrical relaxation time tau_el and, therefore, the thermalrelaxation time tau. The oscillation frequency f_osc_bt of the bridgetype oscillator is then evaluated, as in other embodiments of thepresent invention, in the control and information processing unit 12 inorder to obtain the distance h of the heater element HE from the storagemedium 2.

FIFTH EMBODIMENT

In a further embodiment as shown in FIG. 13, the relaxation oscillatoraccording to the first embodiment shown in FIG. 8 additionally comprisesa phase locking unit. The phase locking unit comprises a phase detector56, a loop filter 58 and an amplifier 60. By way of the phase lockingunit, the relaxation oscillator may be synchronized to an external clock62. The distance h of the heater element HE from the storage medium 2may then be derived from a control signal used for adjusting thefrequency of the oscillator to the reference frequency, which referencefrequency is given by the frequency of a clocking signal CLK generatedby the external clock 62. The phase locking unit is just shown in itsapplication to the relaxation oscillator by way of example. It may,however, also be applied to any of the other embodiments of the presentinvention.

Preferably, the phase detector 56 creates, as output, a phase contrastsignal PH_CON being representative of the phase shift between its twoinput signals. The phase contrast signal PH_CON is then filtered in aloop filter 58. The output of loop filter 58 is then a sensor outputsignal SE_OUT that is amplified in the amplifier 60 and then serves toadjust either the first or the second bias voltage potential V_b1, V_b2.

The loop filter 58 preferably has a pole at zero frequency which assuresa fixed phase relation with respect to the clocking signal CLK, whichpreferably marks a nominal position of a mark represented by thepresence or absence of an indentation. The phase contrast PH_CON may beused to measure the thermal relaxation time tau and, hence, to determinethe distance h of the heater element from the storage medium 2.

The phase locking unit has the effect that, whenever an indentation isprobed during the time interval between two pulses of the clockingsignal, the phase of the oscillator decreases by a constant amount sincethe thermal relaxation time tau is slightly smaller than an average andthus the electrical relaxation time tau is slightly slower than anaverage as well. Conversely, the phase of the oscillator increases by aconstant amount, if no indentation is probed between two consecutivepulses of the clocking signal CLK. Hence, that the digital informationprobed by the heating element HE is encoded in an integrated fashion inthe phase contrast, the signal PH_CON may then be evaluated accordinglyin the control and information processing unit 12 in order to obtain theactual distance h of the heater element HE from the storage medium 2.Indentations may, for example, represent logic variables and may, inthat way, be detected as they correspond to jumps in the phase contrastsignal PH_CON.

By choosing the parameters of the loop filter 58, the sensor outputsignal SE_OUT may represent a check sum over a bit string represented bythe marks and may in that way be exploited for error correction alreadyin a read channel. In this context, it is preferred that the data storedon the storage medium 2 and being represented by the marks 20, 22 and 24is encoded in a balanced code meaning that a given amount of consecutivebits always has a given check sum. For this purpose, the parameters,especially the response time of the loop filter 58, is chosen in a waythat is preferably at least as long as the time it is needed to scanover the given amount of bits in a bit-sequence rendering a given checksum. Typically, such balanced codes are balanced in their check sum overaround 10 to 20 bits.

Another significant advantage of using the phase contrast signal PH_CONis the fact that the external clock signal CLK and the exact on trackposition of the respective marks 20, 22 and 24 do not need to perfectlymatch, which makes the sensing more robust against clocking signal CLKfluctuations.

In FIG. 14, the heater element electrical current I_HE and the heaterelement potential difference U_HE are plotted over the time t for anoperation of the relaxation oscillator according to FIG. 8. They areplotted for a heater element with room temperature electrical resistanceof around 30 k-Ohm which rises to values in the order of 60 k-Ohm in theplateau region of the current/voltage interrelationship curve. Thesteady state oscillation is 280 kilohertz. For that purpose, the thermalrelaxation time tau of the heater element is in the order of 5microseconds. The bias voltage potentials are, by way of example, forthe first bias voltage potential V_b1 around 8 volt and for the secondbias voltage potential V_b2 around 2.5 volts corresponding to steadystate current I_DC of the order of 140 micro Ampere and 75 micro Ampere,respectively. The first and second electrical current threshold I_THD₁,I_THD_2 may in this case be chosen to be around 170 micro Amperes andrespectively 60 micro Amperes.

FIG. 15 shows the power spectral densities of the relaxation oscillatoraccording to the first embodiment of the present invention, which isdenoted by 66, and the power spectral density 68 when applying thepreviously-proposed measuring method of directly measuring theresistance of the heater element. This plot demonstrates thatsubstantially improved 1/f noise characteristics are obtained with anembodiment of the present invention.

In an embodiment of the present invention, the control and informationprocessing unit 12 is designed for detecting output signals of arespective circuit and then deriving the respective distance h of theheater element HE from the storage medium 2.

In an embodiment of the present invention, tau ranges from 0 to 1second, and preferably from 0.1 microsecond to 0.1 millisecond.

Embodiments of the present invention have been explained above inrelation to a storage device, in particular for storing data, by way ofexample. They are however not limited to the specific embodiments. Itwill be understood that the present invention has been described purelyby way of example, and modifications of detail can be made within thescope of the invention.

Each feature disclosed in the description and (where appropriate) theclaims and drawings may be provided independently or in any appropriatecombination.

1. A probe device for sensing topographical variations on a referencemedium comprising: a heater element with a temperature dependentelectrical resistance disposed at a tip of said probe device:measurement means for measuring a parameter associated with a thermalrelaxation time for said heater element; and position determining meansfor determining probe position based on said measured parameter.
 2. Thedevice according to claim 1, further comprising a relaxation oscillatorcircuit, said relaxation oscillator circuit being operable to apply abias voltage potential to said heater element
 3. The device according toclaim 2, wherein said relaxation oscillator circuit further comprises acurrent threshold switching unit operable to perform a first switchingact of switching said bias voltage potential from a first bias voltagepotential to a second bias voltage potential when a first currentthreshold is reached and to perform a second switching act of switchingsaid second bias potential to said first bias voltage potential when asecond current threshold is reached.
 4. The device according to claim 3,wherein said current threshold switching unit is configured to performmultiple of said first and second switching acts consecutively.
 5. Thedevice according to claim 4, wherein said parameter is a switchingfrequency of said switching unit.
 6. The device according to claim 3,wherein said current threshold switching unit is configured to performone of said first and second switching acts.
 7. The device according toclaim 6, wherein said parameter is the duration of time from applyingone of said first and second bias voltage potentials to reaching thecorresponding said first or second electrical current thresholds.
 8. Thedevice according to claim 3, wherein said first current threshold isgreater than a steady state current corresponding to the first biasvoltage potential and said second current threshold is less than asteady state current corresponding to the second bias voltage potentialand said first bias voltage potential is greater than said second biasvoltage potential.
 9. The device according to claim 1, furthercomprising an LC-resonating circuit, the capacitive element thereofbeing said heater element, and wherein said parameter is a resonancefrequency of said LC-resonating circuit.
 10. The device according toclaim 1, further comprising an RC delay line oscillator circuit andwherein said parameter is an oscillation frequency of said RC delay lineoscillator.
 11. The device according to claim 1, further comprising abridge type oscillator circuit and wherein said parameter is anoscillation frequency of said bridge type oscillator.
 12. The deviceaccording to claim 1, further comprising a circuit associated with saidheater element and a phase-locking unit for synchronizing said circuitto an external clock.
 13. The device according to claim 12, wherein saidparameter is a phase contrast signal output from said phase-lockingunit.
 14. The device according to claim 13, said external clock beingoperable to create a clocking signal corresponding to consecutive markpositions on said reference medium when a surface thereof is scanned bysaid probe.
 15. The device according to claim 12, whereby an overallloop gain is controlled so that the time it takes to establish a phasematch is sufficiently long for a balance string of marks to be probed.16. The device according to claim 1, wherein said heater element isoperated in a plateau region of a current/voltage interrelationshipcurve of said heater element.
 17. A method for sensing a position of aprobe relative to a reference medium, said probe comprising a heaterelement with a temperature dependent electrical resistance comprisingthe steps of: measuring a parameter associated to a thermal relaxationtime of said heater element; and determining probe position based onsaid measured parameter.
 18. A device for sensing a position of a proberelative to a reference medium, said probe comprising a heater elementwith a temperature dependent electrical resistance, comprising:measurement means for measuring a parameter associated with a thermalrelaxation time for said heater element; and position determining meansfor determining probe position based on said measured parameter.